EP0037413B1

EP0037413B1 - Strip transmission line tuner circuit

Info

Publication number: EP0037413B1
Application number: EP80902089A
Authority: EP
Inventors: Adel Abdel Moneim Saleh
Original assignee: Western Electric Co Inc
Current assignee: AT&T Corp
Priority date: 1979-10-11
Filing date: 1981-04-21
Publication date: 1986-04-23
Also published as: JPS647681B2; CA1136300A; EP0037413A4; EP0037413A1; DE3071569D1; US4267532A; JPS56501346A; WO1981001080A1

Abstract

The present invention relates to a class of adjustable microstrip and stripline tuners. An exemplary tuner comprises a pair of tuning elements (101, 102), where each tuning element comprises a pair of parallel spaced-apart conductive strips (121, 141; 122, 142) of equal length and at least one movable bridging wire (191, 192) connecting the two strips. The movement of the bridging wire will vary the output impedance of the tuning element, and a complementary arrangement of a pair of tuning elements will form a tuner capable of matching any impedance falling within the Smith chart.

Description

The present invention relates to a strip transmission line tuner circuit comprising a pair of tuning elements disposed over a ground plane, each tuning element comprising a first strip of conductive material and a second strip of conductive material, said second strip being equal in length to and positioned in parallel, spaced apart . relationship with the first strip, the second strip of one of said tuning element being connected in series to the first strip of the other tuning element thereby forming an extended conductive strip having the first strip of the said one tuning element and the second strip of the said other tuning element disposed on opposite sides thereof and in longitudinally spaced relationship thereto. Such a circuit is disclosed in an article "Synthesis of a Class of Strip-Line Filters" by H. Ozaki et al in IRE Transactions on Circuit Theory, Vol. CT-S No. 2, June 1958, at pages 104-109.
Various microwave devices require the use of adjustable tuners in experimental evaluation of their performance. In the past, microstrip and stripline test fixtures were equipped with transitions to coaxial transmission lines and, therefore coaxial-line multi-slug or multi-stub tuners could be employed. However, the large separation between the device and the tuner limited its use to frequencies less than 10 GHz. There is a need, therefore, for tuners that may be employed directly with the microstrip and stripline medium to overcome the frequency limitation of the coaxial-line tuners.
One type of tuner that is available for use with microwave transmission lines is disclosed in U.S. Patent 2,757,344. In such known arrangement, a transmission line comprises a wide and a narrow conductor mounted in parallel on opposite sides of a substrate. A tuning element comprises a first and a second conductor disposed in spaced-apart parallel relationship to each other and normal to the narrow conductor of the transmission line. The ends of the tuning element first and second conductors adjacent the narrow line conductor are coupled thereto, and a coupling means is disposed between and in contact with the first and second conductors. The coupling means is longitudinally movable between the first and second adjacent conductors of the tuning element at a distance from the narrow line conductor, and this coupling means forms, in conjunction with the wide conductor of the transmission line directly adjacent the tuning element, and adjustable resonant network.
The design of stripline filters and directional coupling arrangements are discussed in an article "Coupled-Strip-Transmission-Line Filters and Directional Couplers" by E. M. T. Jones et al in IRE Transactions on Microwave Theory and Techniques, Vol. MTT-4, No. 2, April 1956 at pp. 75-81. There, low-pass, band-pass, all-pass and all-stop basic filter characteristics are obtained from a pair of parallel, spaced-apart, strips either by placing open or short circuits at two of the four available terminals pairs, or by interconnecting two of the terminal pairs. The article further describes how desired performance may be achieved by cascading several of the basic filter sections.
Another design method for a class of stripline filters is discussed in the aforesaid Ozaki et al article which discloses a method relating to design on an insertion loss basis. Synthesis procedures are presented for the line type, low-pass ladder, high-pass ladder and band-pass ladder filter arrangements. The Ozaki et al arrangements comprise line or ladder cascaded canonical filter sections with each strip section comprising a pair of parallel, spaced-apart, strips having either the same or different widths.
Although not concerned with microstrip or stripline devices, in U.S. Patent No. 2,247,779, there is disclosed a high frequency transmission line structure which includes a pair of spaced apart tubular conductors having a movable shorting member connected therebetween to effectively adjust the length of the transmission line and thereby adjust the surge impedance of the line.
The problem remaining in the prior art is to provide a class of tuners which are capable of being formed directly on the microstrip or strip- line medium so as to be close to the device being tested, thereby to increase the usable frequency range of the tuner, and also which are capable of matching any impedance falling within the Smith chart.
This problem is solved in accordance with the present invention in a strip transmission line circuit as aforesaid by arranging that each tuning element comprises a pair of bridging wires connecting its respective strips of conductor material and providing a shunt connection therebetween, said wires being capable of moving along the entire length of their respective tuning element.
The strip transmission line circuit of the present invention may be formed directly on the substrate, and placed as close to the device being tested as desired without affecting the performance of either the device or the tuner.
Another advantage of the present invention'is to provide a tuner which may be connected to the device either through one port to provide an adjustable shunt reactance or through two ports to provide an adjustable two-port reactive network for the device.
Some exemplary embodiments of the invention will now be described, reference being made to the accompanying drawings, in which:

Fig. 1 is a view in perspective of an exemplary tuning element containing two bridging wires in accordance with the present invention;
Figs. 2 and 4 illustrate two known alternative configurations of a parallel-strip circuit for use in analysis of various tuner arrangements formed in accordance with the present invention;
Figs. 3 and 5 illustrate the equivalent circuits of the known parallel-strip circuits associated with Figs. 2 and 4, respectively, for use in analysis of various tuner arrangements formed in accordance with the present invention;
Fig. 6 illustrates a complete tuner in accordance with an embodiment of the present invention comprising two of the tuning elements of Fig. 1;
Fig. 7 illustrates an all-frequency equivalent circuit of the tuner of Fig. 6;
Fig. 8 illustrates a specific equivalent circuit of the tuner of Fig. 6 for the value of ψ=π/2;
Fig. 9 illustrates a variant of the tuner of Fig. 6;
Fig. 10 illustrates a specific equivalent circuit of the tuner of Fig. 9 for the value of ψ=π/2;
Fig. 11 illustrates another variant of the tuner of Fig. 6;
Fig. 12 illustrates a specific equivalent circuit of the tuner of Fig. 11 for the value of ψ=π/2;
Fig. 13 illustrates the Smith chart coverage associated with the tuners of Fig. 9-12;
Fig. 14 illustrates another variant of the tuner of Fig. 6; and
Fig. 15 illustrates a specific equivalent circuit of the tuner of Fig. 14 for the value of ψ=π/2.
Fig. 1 contains an exemplary single parallel-strip tuning element 10 comprising a pair of adjacent, parallel conductive strips of equal length 12 and 14 disposed above a ground plane 13, and a pair of bridging wires 16 and 18 connecting strip 12 to strip 14, bridging wires 16 and 18 being positioned in a manner such that bridging wire 18 is placed to the right of bridging wire 16. Tuning element 10 further comprises four ports 22, 24, 26 and 28, each port disposed at a separate end of strips 12 and 14. For example, ports 22 and 26 are disposed at the left and right ends, respectively, of strip 12 and ports 24 and 28 are disposed at the left and right ends, respectively, of strip 14.

Connecting a single port, e.g., port 22 of tuning element 10 to the device being tested (not shown) enables element 10 to perform as an adjustable single-port shunt reactance, the mobility of bridging wires 16 and 18 accounting for the adjustability of tuning element 10. An adjustable two-port reactive network can be obtained by connecting two ports of tuning element 10 to the device being tested. Each of the remaining unconnected ports of tuning element 10 may be open-circuited or short-circuited. The open circuit configurations are usually preferable because of the inconvenience of creating a short circuit in a microstrip or stripline medium, and because of the possible requirement of maintaining a bias voltage on the transmission line when active devices are involved.
Tuners formed in accordance with the present invention, in order to match any impedance falling within the Smith chart, comprise two tuning elements as shown generally in Fig. 1 and described hereinabove, arranged in a complementary manner as will be described in greater detail hereinafter in association with Fig. 6, 9, 11 and 14. To enable analysis of the tuners formed in accordance with the present invention, Figs. 2-5 illustrate two alternative known parallel strip circuit arrangements and their equivalent circuits which do not include bridging wires, blocks or sliding contacts.
Fig. 2 illustrates a parallel-strip circuit 20 similar to tuning element 10 described hereinabove in association with Fig. 1. Parallel-strip circuit 20 comprises the conductive strips 12 and 14, and ports 22, 24, 26 and 28 associated with tuning element 10 of Fig. 1, but does not contain bridging wires 16 and 18, since wires 16 and 18 are unnecessary in the development of basic circuit configurations. In circuit 20, ports 22 and 24 are connected to form terminal 1 which is available for connection to a utilization circuit (not shown), as are ports 26 and 28 connected to form terminal 2 which is also available for connection to a utilization circuit (not shown).
Fig. 3 illustrates the equivalent circuit 30 associated with parallel-strip circuit 20 of Fig. 2. In accordance with the well-known transmission line theory, the interconnection of ports 22 and 24 and the interconnection of ports 26 and 28, as described hereinabove in association with Fig. 2, creates transmission line equivalent circuit 30 as shown in Fig. 3. The admittance of strip 12 of Fig. 2 is defined as Y₁₂ and the admittance of strip 14 of Fig. 2 is defined as Y₁₄. The admittance of circuit 30, Y₁₂+Y₁₄, is obtained from the application of the well-known 4_X4^ádmittance matrix of parallel-coupled lines, a detailed derivation of which is contained in the article "Even- and Odd-Mode Waves for Nonsymmetrical Coupled lines in Nonhomogeneous Media" by R. A. Speciale in IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-23, No. 11, November 1975 at pp 897-908. The distance ϕ, as shown in Fig. 3, is defined as the electrical length of the equivalent circuit 30. From transmission line theory, ϕ is defined by the well-known relation
where w is the angular frequency of the mode of propagation, I is the physical length of either strip 12 or 14 of parallel-strip circuit 20 of Fig. 2, strips 12 and 14 being of equal length, and v is the velocity of propagation of the mode of propagation.
Fig. 4 illustrates a parallel-strip circuit 21 which is a variant of parallel-strip circuit 20 of Fig. 2. In the case of parallel-strip circuit 21, no connection is provided between ports 22 and '24, port 22 forms terminal 1 which is available for connection to a utilization circuit (not shown), and port 24 is open-circuited. Like parallel-strip circuit 20 of Fig. 2, ports 26 and 28 of parallel-strip circuit 21 are interconnected to form terminal 2.
Fig. 5 illustrates the equivalent circuit 31 associated with parallel-strip circuit 21 of Fig. 4. In accordance with the well-known transmission line theory, the interconnection of ports 26 and 28 and the open-circuit at port 24, as described hereinabove in association with Fig. 4, creates equivalent circuit 31 as shown in Fig. 5. The impedance of strip 12 of Fig. 4 is defined as Z₁₂ and the impedance of strip 14 is defined as Z₁₄. Further, the configuration of strips 12 and 14, in accordance with the present invention, yields the following relations:
where Y₁₂ and Y₁₄ are the admittances as described hereinabove in association with Fig. 3.
Equivalent circuit 31 comprises a series impedance formed by a short-circuited transmission line of characteristic impedance
/(Z₁₂+Z₁₄) in cascade with another transmission line of characteristic admittance Y₁₂+Y₁₄. Both transmission lines have an electrical length φ, which may be obtained by employing equation (1 ).
Fig. 6 illustrates an exemplary tuner formed in accordance with the present invention comprising two tuning elements 10₁ and 10₂, each tuning element being as described hereinabove in association with Fig. 1. Tuning elements 10, and 10₂ share the conductive strip 14, with the portion designated 14, being the half of strip 14 associated with tuning element 10, and the portion designated 14₂ being the half of strip 14 associated with tuning elements 10₂. Strips 12, and 12₂ are positioned on opposite sides of, and parallel to, strip 14; strip 12, being associated with tuning element 10, and strip 12₂ being associated with tuning element 10₂. Bridging wires 16, and 18, interconnect strips 12, and 14₁, and in a like manner, bridging wires 16₂ and 18₂ interconnect strips 12₂ and 14₂.
The electrical lengths φ₁, θ₁, φ₂, θ₂ and ψ can be obtained by using equation (1), where the length I of equation (1) is associated with each of the above-mentioned electrical lengths in the following manner: for φ₁, I is defined as the distance measured between port 22, and bridging wire 16₁, for θ₁, I is defined as the distance measured between port 26₁ and bridging wire 18₁; for φ₂, I is defined as the distance measured between port 22₂ and bridging wire 16₂; for θ₂, I is defined as the distance measured between port 26₂ and bridging wire 18₂; and for ψ is defined as the entire length of either strip 12₁ or 12₂.
Each of tuning elements 10₁ and 10₂ is divided into three cascaded sections, tuning element 10₁ comprising cascaded sections 40₁ and 40₂ and 40₃, and tuning element 10₂ comprising cascaded sections 40₄, 40₅ and 40₆. Each separate section may be analyzed by comparing the separate sections with parallel- strip circuits 20 and 21 of Figs. 2 and 4, where the port interconnections of parallel- strip circuits 20 and 21 serve to perform in a like manner to bridging wires 16₁, 18₁, 16₂, and 18₂ of the tuner of Fig. 6. In the arrangement of Fig. 6, sections 40₁ and 40₄ can be seen to be similar to parallel-strip circuit 21 of Fig. 4 with one end of the parallel-strip sections 40₁ and 40₄ shorted by wires 16₁ and 16₂, respectively, sections 40₂ and 40₅ can be seen to be similar to parallel-strip circuit 20 of Fig. 2 with both ends of sections 40₂ and 40₅ short circuited by wires 16₁ and 18₁ and 16₂ and 18₂, respectively, and sections 40₃ and 40₆ can be seen to be similar to a mirror image of parallel-strip circuit 21 of Fig. 4 with one end of the sections 40₃ and 40₆ shorted with wires 18₁ and 18₂, respectively. The tuner arrangement of Fig. 6 can be seen to comprise six cascaded sections of parallel-strip circuits in accordance with Figs. 2 and 4.
The tuner arrangement may, in turn, be analyzed by employing cascaded sections of equivalent circuits 30 and 31 of Figs. 3 and 5, where equivalent circuits 30 and 31 are associated with parallel- strip circuits 20 and 21, respectively. This analysis is described in greater detail hereinafter in association with Fig. 7.
Fig. 7 illustrates an exemplary all-frequency equivalent circuit associated with the tuner of Fig. 6. As stated hereinabove in association with Fig. 6, Fig. 7 comprises cascaded sections of equivalent circuits 30 and 31 of Figs. 3 and 5. Specifically, the overall equivalent circuit is divided into six cascaded sections, each separate section being of the form of equivalent circuit 30 or 31, as denoted by the numeral accompanying each section, and each separate section also being associated with its respective section of Fig. 6, as denoted by the subscript accompanying each numeral. For example, section 30₁ of Fig. 7 is of the form of equivalent circuit 30 and is related to the first section, 40₁, of the tuner of Fig. 6 between ports 22₁ and 24₁ and bridging wire 16₁, and section 31₅ of Fig. 7 is of the form of equivalent circuit 31 and is related to the fifth section, 40₅, of the tuner of Fig. 6.
The impedance or admittance of each section of Fig. 7 can be related to the appropriate section of Fig. 6 in the following manner: Z1₂ and Y¹ ₁₂ are associated with the portion of strip 12₁ associated with section 40₁,
and
are associated with the portion of strip 14₁ associated with section 40₁,
and
are associated with the portion of strip 12, associated with section 40₂, and continuing in a like manner such that Z⁶ ₁₄ and
are associated with section 40₆ of strip 12₂.
The notation may be simplified by the following reductions:
The arrows shown on the series impedance sections of the equivalent circuit of Fig. 7 are to illustrate the variability of these elements caused by the variations in φ₁, 8₁, φ₂ and 8₂ due to the movement of bridging wires 16₁, 18₁, 16₂ and 18₂, respectively. Note that the overall lengths of the cascaded transmission line sections φ₁+α₁,+θ₁ and φ₂+α₂+θ₂, each of which being equal to ψ, do not change, since ψ is the electrical length of the entire tuning element, which cannot be varied.
The variability of the equivalent circuit will be discussed in greater detail hereinafter in association with Fig. 8.
Fig. 8 illustrates a specific equivalent circuit of the all-frequency equivalent circuit of Fig. 7 depicted for the value of ψ=π/2. The specific value of ψ is chosen for illustrative purposes only and is not intended to limit the scope and spirit of the present invention. Using the value of ψ in association with the relations
where Z₁₂=Z¹ ₁₂=Z³ ₁₂=Z⁴ ₁₂=Z⁶ ₁₂ in the present example if strips 12, and 12₂ are symmetric, in association with well-known definitions from transmission line theory, the equivalent circuit of Fig. 7 may be reduced to the specific equivalent circuit of Fig. 8. The specific circuit comprises four adjustable active elements, L₁, L₂, C₁ and C₂, where each element is defined as follows:

where w is the angular frequency, and where each separate element is a function of one of the four electrical lengths φ₁, θ₁, ϕ2 or θ₂.
It can be shown from well-known basic circuit theory techniques, that independently varying the values of φ₁, 8₁, φ₂ and 8₂ from 0 through π/2 by the movement of bridging wires 16₁, 18₁, 16₂ and 18₂, respectively will allow this equivalent circuit, and hence the tuner of Fig. 6, to be capable of matching any impedance falling within the Smith chart.
Fig. 9 illustrates a variant of the tuner of Fig. 6 where bridging wires 16, and 16₂ are positioned at the extreme left ends of tuning elements 10, and 10₂, respectively, thereby setting φ₁=φ₂=0. Therefore, under such conditions, only the movement of bridging wires 18, and 18₂ are capable of affecting the performance of the tuner.
Fig. 10 can be derived from Fig. 8, where in this case jrY_ctanφ₁=jωL₁=0 and jrY_ctanφ₂=jωC₂=0, since φ₁=φ₂=0 as shown hereinabove in association with Fig. 9. The equivalent circuit of Fig. 10, therefore, contains only two of the adjustable active elements of the circuit of Fig. 8, C₁ and L₂, which are functions of the distances θ₁ and θ₂, respectively. Varying the values of θ₁ and θ₂ from 0 through π/2 by the movement of bridging wires 18, and 18₂ will cause the tuner associated with Fig. 9 to be capable of matching exactly half of the impedance values falling within the Smith chart.
Fig. 11 illustrates another variant of the tuner of Fig. 6 where bridging wires 18, and 18₂ are positioned at the extreme right ends of tuning element 10₁ and 10₂, respectively, thereby setting θ₁=θ₂=0. Therefore, under such conditions, only the movement of bridging wires 16, and 16₂ are capable of affecting the performance of the tuner.
Fig. 12 illustrates the equivalent circuit of the tuner of Fig. 11 for the value of ψ=π/2. This equivalent circuit is similar to the circuit of Fig. 8, where in this case jrY_ctanθ₁=jωC₁=0 and j(r/ Y_c)tanθ₂=jωL₂=0, since θ₁=θ₂=0 as shown hereinabove in association with Fig. 11. The equivalent circuit of Fig. 12, therefore, contains only two of the adjustable active elements of the circuit of Fig. 8, L₁ and C₂, which are functions of φ₁ and φ₂, respectively. Varying the values of φ₁ and φ₂ from 0 through π/2 by the movement of bridging wires 16₁ and 16₂ will cause the tuner of Fig. 11 to be capable of matching the impedances within the Smith chart not matched by the tuner of Fig. 9.
Fig. 13 illustrates the Smith chart coverage referred to hereinabove in association with Figs. 10 and 12. The darker half of the Smith chart is associated with the tuner of Fig. 9, and the lighter half of the Smith chart is assocaited with the tuner of Fig. 11. Therefore, the combined use of the pair of tuners of Figs. 9 and 11 will be capable of matching any impedance falling within the Smith chart.
Fig. 14 illustrates another variant of the tuner of Fig. 6. In this case, bridging wires 16₁ and 18₁ are merged to form a single bridging wire 19₁, likewise, bridging wires 16₂ and 18₂ are merged to form a single bridging wire 19₂. The distances φ₁, θ₁, φ₂ and θ₂ are redefined as follows: φ₁ is defined as the electrical length measured between port 22₁ and bridging wire 19₁, calculated by using equation (1) where I is the physical length measured between port 22₁ and bridging wire 19₁. In a like manner, φ₂ is defined as the electrical length measured between port 22₂ and bridging wire 19₂, calculated by using equation (1) where I is the physical length measured between port 22₂ and bridging wire 19₂. The distance θ₁ is defined as the electrical length measured between port 26₁ and bridging wire 19₁, calculated by using equation (1) where I is the physical length measured between port 26₁ and bridging wire 19₁. Likewise, the distance θ₂ is defined as the electrical length measured between port 26₂ and bridging wire 19₂, calculated by using equation (1) where I is defined as the physical length measured between port 26₂ and bridging wire 19₂. The distances, as seen in Fig. 14 are interrelated as follows:
The interdependence of φ₁ and θ₁, and of φ₂ and 8₂ will be discussed in greater detail hereinafter in association with Fig. 15.
Fig. 15 illustrates the equivalent circuit of the tuner of Fig. 14. The four adjustable active elements L₁, C₁, L₂ and C₂ are as described hereinabove in association with Fig. 8. In this case, however, the four elements are not independent, rather L, and C₁ are interdependent and L₂ and C₂ are interdependent as shown by the dotted lines in Fig. 15. This interdependence can be determined by referring to Fig. 14, where increasing can be seen to decrease θ₁. Similarly, increasing φ₂ can be seen to decrease 8₂. Therefore, the value of L₁, j(r/Y_c)tanφ₁, varies inversely proportional to C_i, jrY_ctanθ₁. Similarly, the value of L₂, j(r/Y_c)tanθ₂, varies inversely proportional to C₂, jrY_ctanφ₂. Due to this interrelationship, varying the placement of bridging wires 19, and 19₂ will cause the tuner of Fig. 14 to be capable of matching any impedance falling within the Smith chart.
It is to be understood that the structures depicted in Figs. 9, 11 and 13 are described as being variants of the embodiment of Fig. 6 and as such are each deemed to include the movable bridging wires 16₁, 16₂, 18₁ and 18₂ thereof, even though in Figs. 9 and 11 various ones of the bridging wires are shown in preset positions and even though in Fig. 13 pairs of the bridging wires are merged to form a single bridging wire. Thus, the structures of Figs. 9, 11 and 13 do not themselves constitute different embodiments of the invention claimed.

Claims

A strip transmission line tuner circuit comprising a pair of tuning elements (10₁, 10₂) disposed over a ground plane, each tuning element comprising a first strip of conductive material (121, 14₂) and a second strip of conductive material (14₁, 12₂), said second strip being equal in length to and position in parallel, spaced apart relationship with the first strip, the second strip (14₁) of one of said tuning elements (10,) being connected in series to the first strip (14₂) of the other tuning element (10₂) thereby forming an extended conductive strip (14) having the first strip (12,) of the said one tuning element (10₁) and the second strip (12₂) of the said other tuning element (10₂) disposed on opposite sides thereof and in longitudinally spaced relationship thereto, characterised in that each tuning element (10₁, 10₂) comprises a pair of movable bridging wires (16₁, 18₁, 16₂, 18₂) connecting its respective strips of conductive material and providing shunt connection therebetween, said wires being capable of moving along the entire length (ψ) of their respective tuning element.